Purpose of this Posting:
This posting is intended to show the investigation/analysis of the radiator transmission cooler fittings connection for one failure on 2003 Acura MDX. From reading MDXers.org I have been made aware of the issue that has shown up recently for cars of this age/mileage, I was able to diagnosis it once I saw the radiator overflow tank. I just wanted to add more information to the collective knowledge.Automobile: Acura MDX (Nighthawk Black Pearl, Base, 2003, 93K miles)Location: Naperville, IL (Suburb of Chicago, rust-belt)Maintenance: Radiator Coolant factory installed Symptoms (November 2011):
• Transmission slippage in all gears that progressively got worse over two days.
• RPMs not matching transmission gear and vehicle speed.
• Found “Strawberry Milkshake” in the radiator overflow tank.Diagnosis: Failure of transmission cooler inside of radiator.Repair: Tow car to home, remove and replace radiator. Analysis of the failure:
• The radiator is a top to bottom coolant flow. The automatic transmission fluid flow is side to side inside the transmission cooler which is inside of the bottom tank of the radiator, immersed in coolant.
• The failure/leakage was internal to the radiator. All external hoses and fittings were still attached when the radiator was removed.
• The inlet and outlet fitings for the transmission cooler (TC) were easily broken off with a little pressure . The radiator and the TC line fittings are shown below:

• The aluminum body of the radiator (core) was removed from the bottom (tank) by prying up all the aluminum tabs that crimp the two pieces together. There is a gasket between the core and the tank.
• Below is a picture of the bottom black radiator tank (coolant) and the aluminum internal transmission cooler (automatic transmission fluid).

• Below is one of the female sections of the threaded fittings on the internal transmission cooler. The other section is very similar to the pictured one.
Note:
• The size of the O-rings compared to the outer flange.
• The amount of corrosion still left after initial cleanup.
• The outer diameter of the inner connection is just slightly larger than the hole in the radiator tank. Since the inside is not able to slide completely into the hole, the outer flange holding the O-ring is held up about 1/32’’ away from the inner wall of the radiator tank. The O-ring is then mainly responsible for keeping the coolant out of the connection.

• Below are the male sections of the threaded fittings.
Note:
• The amount of corrosion on the outer flange.
• The amount of the corrosion on the threaded portion (white hard crusty, lime scale??).
• The red/orange thread lock material seen on only 1 fitting.
• That the inner aluminum tubing and O-rings were in good condition

At this point, I thought the outer tubing, hex nut, and circular flange were all one piece made of aluminum. However, the corrosion between the hex nut section and the circular flange/washer just did not look right to me. It showed flakiness that is normally seen only in rusted steel. I was able to separate it into 3 parts (as shown below):
1. Aluminum tubing with hex nut section.
2. One rusted steel washer in the middle.
3. One rusty steel washer on the outside.

Conjecture on what went wrong with the connection:
• Corrosion around the threads weakened the connection.
• The corrosion could have come from two places.
• Water (and salt, being in the rust belt) can get between the steel washers and the tank and eventually enter the threaded area. There are no O-rings on the outside.
• Coolant might have slipped past the internal large O-rings and also entered the threaded area.
• The internals to the transmission cooler looked good, The end of the cooler fitting had no corrosion and a tight small O-ring.
• The threaded connection eventually starts leaking internal to the radiator.
• Due to the higher fluid pressure, the ATF is mainly forced into the coolant.
• The two steel washers on the outside of the radiator experienced very different corrosion. The washer in the middle that touched the aluminum on one side had major corrosion that might have been caused by galvanic corrosion due to the dissimilar metals (steel and aluminum). The washer closest to the radiator tank was in much better condition.
• The amount of threads that are engaged between the fittings and the transmission cooler core is also a concern. It looks like only 2 rows were ever engaged looking at the fitting. I was able to count at least 5 rows of threads when the corrosion was scraped away. Looking at the threads in the core, it was unclear how many were actually engaged.
• Could the outside washers have prevented the threads from fully being more fully seated?
• Why are the O-rings that seal out the coolant larger than their containing flange?

Summary: The connection of the tube fitting to the transmission cooler had the following issues:
1. No corrosion protection on the steel washers.
2. Potential for galvanic corrosion with steel and aluminum washers placed side-by-side.
3. Threaded mechanism with potential issues.
4. Internal O-rings with a significantly different diameter afterwards.

Prevention:
The reliability of the new radiator relies mainly upon the manufacturer. The only thing I was able to add, was to spray paint the exposed transmission line fitting hardware in an attempt to prevent future corrosion. It is unclear how many of the issues that I had with the previous radiator still appear in the design of the new one. It is also unclear to me, if this could have been prevented with a more timely coolant change.

All:
I replaced the radiator with an aftermarket radiator. It was chosen for availability, price, and perceived quality. Since, I needed the MDX up and running, I was not able to do or find much research on the quality and reliability of a new radiator. Allow me not to name the brand as I do not want to endorse one brand over another based upon my limited knowledge. The new radiator had different hardware for the TC fittings as compared to the OE.

I agree that it is unclear if any routine maintenance could have prevented this as supported by the anecdotal evidence as reported in these forums.

Thanks for the links to the other forums. The pictures are very similar. Below is a quote from one of the Ridgeline threads:>> My truck has what appears to be a thin stainless steel washer and shows no signs of corrosion. A dissolving steel washer could certainly swell to the point where it pulled out the threads and that's what appears to me to be what happened.
The washers on my radiator definitely showed the same corrosion, one badly rusted washer, one in much better condition.

The main point of the quote was that a steel washer experiencing severe corrosion could "swell" enough to pull on and damage the internal threaded fittings. Time for me to do some more research on the web.

Thank you for these pictures, and the analysis. I am not clear: are the cooler line nipples simple threaded connections to the cooler that could be loosened periodically for inspection or even replacement or re-sealing? I have an '05 with 75,000 miles and I'm getting ready to do a cooler bypass with an auxilliary cooler to remove all doubt about the factory cooler, but if the cooler's typical failure point is actually this threaded connection, rather than the physical cooler itself inside the radiator body, it would seem like an inspection of the connection could be made.

Good point Hokiefyd. If the connections are visible and can be inspected for corrosion (especially the corroded steel washer) then perhaps this failure could be avoided. I have a 2005 MDX with 158,000 km and am worried. Please let me know how the cooler bypass system can be installed and who to buy the parts from.
Thanks.

Thank you for these pictures, and the analysis. I am not clear: are the cooler line nipples simple threaded connections to the cooler that could be loosened periodically for inspection or even replacement or re-sealing? I have an '05 with 75,000 miles and I'm getting ready to do a cooler bypass with an auxilliary cooler to remove all doubt about the factory cooler, but if the cooler's typical failure point is actually this threaded connection, rather than the physical cooler itself inside the radiator body, it would seem like an inspection of the connection could be made.

The connection from the:
- cooler line nipples
- to the cooler line is a threaded connection, with thread-lock on one nipple with the 90 degree bend.

However, this connection holds the large O-ring tight against the inside wall of the radiator bottom tank. See Pic #3 above. So, if you wanted to inspect the connection by unthreading, you will have to drain the radiator of coolant and the cooler of transmission fluid. Replacement of the thread-lock material would also be needed. IMHO, I would not try this. I think there are too many things that could go wrong especially with the large internal O-ring.

I can't figure how the fluids are mixing if it is just the cooler fittings, seems like it should just dump the tranny fluid but not the radiator fluid at all?

The cooler fitting connection is inside of the transmission cooler which is inside of the radiator bottom tank. Once the transmission fluid has breached the thread connection, it only has to get past the large O-ring in (Pic #3) to enter the coolant. The O-ring is held in place by pressure from the the connection that was just breached.

Another way to think about is that the cooler line fitting hex section is sitting right next to the outside wall of the radiator tank(w/ 2 thin washers). The threads and small O-ring are inserted through the hole in the radiator bottom tank and into the female section of the cooler. The threads and small O-ring are about 1/2 in long and resides effectively on the inside of the cooler which is inside of the radiator. (See Pic #2)

If the explanation above does not make sense, I can add more pics or add more arrows to the existing pics.

Good point Hokiefyd. If the connections are visible and can be inspected for corrosion (especially the corroded steel washer) then perhaps this failure could be avoided. I have a 2005 MDX with 158,000 km and am worried. Please let me know how the cooler bypass system can be installed and who to buy the parts from.
Thanks.

I definitely agree that inspections are warranted for this connection.
If a transmission cooler bypass system is installed, the bad connection in the radiator still remains holding that O-ring in place. The connection could still fail at a later time, but only affecting the radiator.

However, this connection holds the large O-ring tight against the inside wall of the radiator bottom tank. See Pic #3 above. So, if you wanted to inspect the connection by unthreading, you will have to drain the radiator of coolant and the cooler of transmission fluid. Replacement of the thread-lock material would also be needed. IMHO, I would not try this. I think there are too many things that could go wrong especially with the large internal O-ring.

After further understanding of the design from your subsequent posts, I definitely agree with you. I took a look at mine tonight and it's very hard to make a proper inspection. Pulling the under-body air panel down wouldn't have gained a better view. I used a small mirror to look at the connections, and I can see some light corrosion at the washer behind the hex head of the pipe. It doesn't look severe at this point, but I will move to replace the radiator soon. We are taking a trip with the car this weekend, and will likely replace the radiator next week.

The same radiator fittings on a Honda Pilot were mentioned in the Chicago Tribune:

Q: My daughter’s 2005 Honda Pilot with 64,000 miles blew the transmission fluid line out of the radiator, fitting and all. Is this a common problem? The bill came to $800, so it is not a small problem.
- R.C. , Glastonbury, Conn
A: Compared to what could have happened, you may want to consider this a small problem. Honda has had transmission issues recently and this seems to be one, but is is even worse if the transmission fluid leaks into the coolant. When the two mix into what looks like a strawberry milkshake, your transmission and radiator are toast, and that will be a big bucks repair. The engine will also have to be flushed. We have not head of many owners getting goodwill fixes from Honda.

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